The breathability of a garment is a measure of how well the fabric can transport sweat vapor away from the wearer’s body, and dispel it into the air. Many hardshells are both waterproof and breathable; they keep the liquid water droplets outside on the surface of the fabric, while simultaneously transporting sweat vapor to the outside. This is the job of materials like Gore-Tex; keep you dry from the outside, and the inside. No garment is perfectly breathable; if you work (or play) hard enough, eventually your heat and sweat will overcome the breathable membrane, and start to feel clammy or wet on the inside, even though the fabric is still waterproof, and is not leaking. The end goal is to make a jacket as waterproof as a sheet of steel, but breathe like a cotton T-shirt. Some companies, in recent years, have started coming pretty close to the end goal, with fabrics like eVent, and Montbell’s Breeze Dry-Tec fabric.
Breathability is usually measured in terms of how much vapor can pass through a square meter of fabric in 24 hours. It looks like this: g/24hrs./m2. For example, a pretty good rating for the breathability of a fabric is 15,000g/24hrs./m2. One of my hardshells, a Gore-Tex jacket, is rated at 30,000g/24hrs./m2. It’s pretty good. Other fabrics may be rated under 10,000g/24hrs./m2. But is a breathability rating of 5000g/24hrs./m2 six times worse than my Gore-Tex shell? Probably not. These tests are conducted in the lab, and they don’t really take into account dirt and changing weather conditions and humidities, so the numbers have a certain amount of subjectivity; not every company tests their jackets the same way. Generally, though, the higher the number, the better the breathability of the fabric.